"Subjunctive" is by far used more often in Portuguese than it is in English. It may be a little tricky for English native speakers. Subjunctive is a subordinate clause that expresses doubt, hypothetical situations, wishes and orders, and are followed by QUE. it can be used in past, present or future. The subordinate clause may start with para/a fim de/ antes /sem / ainda / contanto / mesmo / desde / creio... + QUE (most for present) // SE if it is past and QUANDO for future tense (ps: That's a general rule. To learn more you have to dig deeper) =)
Yes if you even say the word "subjunctive" to most English natives (myself included before Duolingo), they'll ask you what language you're speaking!
So (and please correct me if i'm wrong) even though the literal translation may be "He died before I arrived." Would the actual meaning of this sentence in English be "He would have died before I could have arrived"? The reason for my confusion on this one is that the English translation given does not express any doubt.
Actually, the use of subjunctive is not just to express doubt. In Portuguese this sentence does not express doubt either. One of the uses of subjunctive is "hypothetical situations". In that sentence the person died before the second one arrived. But who could guarantee that the person would be still alive if he/she had arrived earlier?. Another point is that it expresses WISHES (what is more rational in this sentence). So, the person wanted to be there before he died. (SE eu CHEGASSE antes, ele não teria morrido -- if I HAD ARRIVED earlier, he wouldn't have died -- in both languages it expresses a wish that no longer can come true) -- "ele morreu antes que eu chegasse" that happened (undoubtedly) but you did not want to
I believe that "doubt" is not a good parameter to think about subjunctive. "Hypothesis" is!
It's very hard for us natives to explain this, because we probably have never thought about that. And english simply condenses everything into 3 tenses helped by auxiliaries.
But I believe the main thing here is the "Hypothesis" of arriving "on time" (to see him die, or to do anything that was to be done before he died, like trying to save, to examine or to say good bye).
So, the arrival on time is an Hypothetical situation that didn't happen, that's why subjunctive is used.
And if you tried indicative here (antes que eu cheguei), that would be a great mess. No meaning, just a clearly recognizable mistake.